This invention relates to hot dip coating of continuous metal strip with molten metal. More particularly, the invention relates to an improved bearing system for supporting a submerged roll in the bath of the molten coating metal.
Wear resistance has been improved in recent years for steel rolls used on high speed hot dip coating lines. It is common practice for furnace hearth rolls and submerged pot rolls to be overlaid with a protective ceramic coating of oxide, carbide or nitride by known thermal deposition techniques. Dross pickup from the surface of the coating bath to submerged pot rolls is minimized because dross particles do not wet the ceramic roll surface as well as a steel roll surface. Similar improvements have not been made, however, in the bearing systems for supporting these rolls. The service life of present day bearing systems does not economically justify the expense of a pot roll overlaid with a ceramic coating.
Bearings are used for supporting submerged rolls in a hot dip molten metal coating pot. U.S. Pat. 5,099,780 discloses using a pair of spaced arms for supporting submerged sink or stabilizing rolls. Each end of a roll includes an axially extending stub shaft having a temperature and wear resistant sleeve mounted thereon. A metal bushing element is welded into an opening provided in the lower end of each support arm. Each bushing element is provided with an axially extending oversized opening for receiving one of the sleeve mounted stub shafts. The bushing element includes two or more flat bearing surfaces disposed in intersecting planes providing point contact between each bushing element and the sleeve. This bushing element configuration eliminates the tendency for a pot roll to chatter when hot dip coating a metal strip. Nevertheless, this roll support system is unsatisfactory because the service life of the bushing element and sleeve may be less than that of the submerged pot roll. Uneven wear to the bushing element and sleeve caused by lateral thrusting of the pot roll increases friction and causes uneven rotation of the pot roll. Uncontrolled lateral thrusting of the pot roll also causes wear or galling to the stub shafts.
World patent application 91-3581 also discloses a support system for a submerged pot roll. Each end of the pot roll includes an axially extending trunnion having a sleeve mounted thereon. The roll is supported by a pair of spaced arms with the lower end of each arm provided with a housing for receiving one of the sleeve mounted trunnions. Each housing includes a chevron shaped ceramic bearing shell having two flat bearing surfaces disposed in intersecting planes to provide two point contact between each bearing shell and sleeve. The working life of the bearing shell and bushing is increased. Nevertheless, this roll support system is unsatisfactory because a sleeve wider than the bearing surface causes uneven wear when roll thrust occurs thereby resulting in roll chatter.
Accordingly, there remains a need for a bearing support system for a submerged pot roll wherein the support system has a life expectancy at least equal to that of the pot roll to eliminate unplanned coating line interruptions for replacing the support system. There also remains a need for an improved bearing support system for a submerged pot roll to justify the cost of applying a ceramic overlay coating to the surface of the pot roll. There is a further need to control lateral thrusting of the pot roll to eliminate uneven wear to the support system and to prevent galling of the pot roll trunnions.